Rhynchotrema seletensis, F.Nikitin & E.Popov & G.Bassett, 2003

F. Nikitin, Igor, E. Popov, Leonid & G. Bassett, Michael, 2003, Late Ordovician brachiopods from the Selety river basin, north Central Kazakhstan, Acta Palaeontologica Polonica 48 (1), pp. 39-54 : 48-51

Rhynchotrema seletensis sp. nov.

Figs. 5A–F , 6 ; Table 9.

Derivation of name: After the Selety river near the type locality .

Holotype: NMGW 98.30 G.41, complete shell.

Type locality: Sample 2523, Shollakkarasu river, Selety river basin, Kazakhstan.

Type horizon: Ordovician, Tauken Formation.

Paratypes.—486 conjoined valves, 12 ventral and 10 dorsal valves.

Diagnosis.—Shell dorsibiconvex, about 80% as long as wide and 80% as thick as long; ventral valve with weakly developed deltidial plates and sulcus originating slightly posterior to mid−valve length; dorsal valve strongly and slightly unevenly convex in lateral profile with moderately high median fold, trapezoidal in cross section; radial ornament of 11 to 20 ribs with 3 ribs on ventral sulcus and 4 ribs on median fold; ventral muscle field large, extending anterior to mid−length; dorsal median septum slender, about one half as long as the valve.

Description.—Shell dorsibiconvex, slightly transversely suboval with maximum width at the mid−length.Anterior commissure strongly uniplicate.Ventral valve moderately convex in transverse profile with maximum thickness near the mid−valve length.Beak slightly incurved, acuminate.

Deltidial plates weakly developed.Sulcus originates about 4 to 6 mm from the beak, with steep flattened lateral slopes, deepening anteriorly and forming a trapezoidal tongue that occupies about 60% of maximum valve width and is about 37% as high as the shell.Lateral slopes flattened slightly in posterior view.Dorsal valve strongly convex with the maxi − mum thickness slightly anterior to the mid−valve length, about 75% as long as wide with an obtuse, incurved beak. A median fold complementary to the sulcus originates at about 4 to 6 mm from the umbo, trapezoidal in cross section with steep, smooth lateral slopes.Radial ornament costate with 11 to 20 angular ribs, mostly with 3 ribs in the sulcus and 4 ribs on the median fold and 6 to 8 ribs on the flanks.Speci − mens with 15 to16 ribs in total are predominant.Concentric ornament comprises fine, evenly spaced fila at about 8 to 10 per 1 mm, developed occasionally as slightly stronger growth lamellae.

Ventral interior with strong teeth supported by short, dental plates.The floor of the delthyrial chamber is occupied by a pedicle callist.Muscle field large, extending anterior to mid−valve length.A small, elongate central suboval adductor mus − cle track is bounded laterally and anteriorly by large, subflabellate diductor scars.Dorsal valve with a disjunct hinge plate and a small, narrow cruralium supported by a low, thin median septum extending anterior to the mid−valve length. Cardinal process high, blade−like.Crura long, radulifer.

Discussion.—By comparison with North American species of Rhynchotrema , R. seletensis somewhat resembles Rhynchotrema nutrix (Billings, 1866) as revised by Jin (1989), but can be distinguished in having a slightly more transverse shell outline, a dorsal median fold originating at some distance from the umbo, usually exceeding the 3 mm characteristic of the American species, and in the absence of a distinctive median furrow in the umbonal area of the dorsal valve.It differs from Rhynchotrema iowense Wang (1949) from the Maquoketa Formation (Ashgill) of Iowa, as well as from specimens identified as the same species described by Laurie (1991: 92, fig.48A) from the lower Benjamin Lime − stone (Caradoc) of Tasmania in being generally larger and in having a strongly dorsibiconvex shell and well defined dental plates.The Kazakhstanian shells also differ from an − other Tasmanian species, Rhynchotrema ponderosa Laurie (1991: 90 , fig.46B), in having a strongly dorsibiconvex lat − eral shell profile, a well defined dorsal median fold which is prominent in posterior view, well developed dental plates, and a long dorsal median septum extending anteriorly to the centre of the valve.

R. seletensis and Rhynchotrema oepiki Percival (1991: 159 , fig.18) from the Eastonian (middle Caradoc) of New South Wales, Australia are similar in having a strongly dorsibiconvex lateral shell profile in mature specimens, an open delthyrium with very short deltidial plates, and well defined dental plates, but the Kazakhstanian species differs in the consistent absence of rib bifurcation on the median fold, as 0.15 0.25 0.35 0.50 0.65 0.75 0 5 mm

well as having a ventral sulcus and dorsal median fold that originate at a considerable distance from the umbo.

Occurrence.—Localities 397a, 550a, 780, 2523 and 5190.

Figures

Fig.5. A–F. Rhynchotrema seletensis sp.nov. A, C, F, sample 2523; B, D, E, sample 550a. A. NMW 98.30G.40, ventral (A1), dorsal (A2), anterior (A3), and lateral (A4) views of conjoined valves, × 2.25. B. NMW 98.30G.43, ventral internal mould (B1) and latex cast (B2), × 3. C. NMW 98.30G.41, holotype, ventral (C1), dorsal (C2), lateral (C3), and anterior (C4) views of conjoined valves, × 2.25. D. NMW 98.30G.45, latex cast of dorsal interior, × 2.5. E.NMW 98.30G.42, latex cast of dorsal valve interior (E1) and dorsal internal mould (E2), × 2.25. F. NMW 98.30G.44, anterior (F1), ventral (F2), dorsal (F3), and lateral (F4) views of conjoined valves, × 3. G. Dinorthis taukensis sp. nov.; NMW 98.30G.37, holotype, sample 2523; dorsal (G1), ventral (G2), lateral (G3), anterior (G4), and posterior (G5) views of conjoined valves, × 2.25. H. Glyptomena kaskolica sp. nov.; NMW 98.30G.12, holotype, sample 550a; latex cast of ventral interior showing muscle field, × 2.25.

Fig.6.Transverse serial sections of conjoined valves of Rhynchotrema seletensis sp.nov.Distance in mm is measured from the posterior tip of the ventral beak. Dorsal valve uppermost.